What do you fill MDF joints with?
I use Ronseal high performance wood filler. Easy to sand and sands smooth. You'll have to sand between coats of primer because first coat will raise the grain of mdf leaving filled areas non-raised.
And mix in the hardener. Now the rule for the hardener with this. Is if you had a golf ball-sized
So use a half-decent acrylic primer undercoat paint your edge forget about the rest of it for now
Woodworkers use spline joints to join two pieces of like material together. The spline joint is a piece of wood centered between two pieces of MDF to connect them. When glue and clamps are employed, an MDF joint is nearly invisible and almost indestructible, as long as the MDF is thick enough.
My Favorite Nail Filler and How to use It - YouTube
Drywall compound is water based, so if you apply to the edge of MDF, you will have to wait a lot longer to throughly dry (& shrink back down).
wood joint gap fill trick - YouTube
Our recommendation for the best wood filler is the Toupret Extrem Wood Filler, also from Toupret's filler range, as it provides a comprehensive repair system for interior and exterior wood, plus it is designed to repair both large and small gaps, cracks and holes.
To make a versatile wood filler for free, just grab a paper plate and combine Elmer's or any other wood glue with sawdust.
I have used biscuits to join 12mm MDF quite successfully. I just wiggle the jointer a bit to enlarge the slot slightly.
What primer should I use on MDF?
Prime the MDF with a solvent-based primer, such as Zinsser (buy online) or KILZ (buy online). Avoid water-based primer, as it may cause the wood fibers to swell, resulting in a surface that appears to have a raised grain (no matter how much effort you put into sanding).
Glued dowels bond with the MDF fibers to strengthen joints. And unlike screws, dowels can hide below the surface. Of course, there's no margin for error when positioning dowel holes in mating parts. Oversize dowel holes can weaken MDF, so use 1⁄ 4 " dowels for 1⁄ 2 "-thick stock, and 3⁄ 8 " dowels for 3⁄ 4 " stock.
